The present invention relates to a drive circuit for a matrix type liquid crystal display providing a visual display of characters, patterns or the like.
A scheme of circuit construction for driving a matrix type liquid crystal display by a line sequential scanning method is illustrated in FIG. 1. Data contained within a master memory 1 is converted into display pattern signals via a character signal converter 2, stored temporarily within a buffer memory in a column driver 3 and then supplied to column electrodes Y.sub.1, Y.sub.2, . . . Y.sub.n of a matrix type liquid crystal display 6. There is further provided a row driver 4 which scans row lines X.sub.1, X.sub.2, . . . X.sub.m of the matrix type liquid crystal display 6 one by one. A control circuit 5 provides all controls for the master memory 1, the column driver 3 and the row driver 4.
With such a matrix type liquid crystal display driven by the row sequential scanning method, the greater the number of the row electrodes the shorter a period of time of voltage applied per row with respect to a full scanning period, in other words, the so-called duty factor is reduced, presenting a crosstalk problem. This is critical particularly for a liquid crystal display having the inherent properties of a threshold level that is not well defined and a response that is too slow. This results in an insufficient contrast in the liquid crystal display.
Some approaches to resolve this problem have been suggested, for example, to modify a matrix electrode structure so as to increase the duty factor. As seen from FIG. 2 showing an equivalent circuit of the electrode structre, one way is to cause one row electrode X.sub.i to confront two independent column electrodes Y.sub.1j, Y.sub.2j, thereby doubling, of quadrupling, . . . the number of energizable rows without deteriorating a display contrast. It seems possible to develop a matrix type liquid crystal display having as many as 100 rows.
Nevertheless, while a twisted nematic field effect mode liquid crystal display (referred to as "TN-FEM-LCD" hereinafter) can be used with a matrix drive and will assume a comparatively good contrast when viewed from a specific direction, it is disadvantageous in that variations in the viewing direction cause a reduction in contrast and difficulties in recognizing a visual display thereof. For the planar type display, the viewing angle differs to a great extent from the top of the bottom, viz., .theta., and .theta..sub.2, with the resulting difference in contrast from the top to the bottom.
It is therefore an object of the present invention to provide a drive circuit which overcomes the shortcomings with the prior art device, with the feature that a drive voltage is varied in accordance with scanning electrodes in practising the row sequential scanning method.